Essential Oils from Southern Italian Aromatic Plants Synergize with Antibiotics against , and Cell Growth and Biofilm Formation.

The spread of antibiotic-resistant pathogens has prompted the development of novel approaches to identify molecules that synergize with antibiotics to enhance their efficacy. This study aimed to investigate the effects of ten Essential Oils (EOs) on the activity of nine antibiotics in influencing growth and biofilm formation in , , and . The effects of the EOs alone and in combination with antibiotics on both bacterial growth and biofilm formation were analyzed by measuring the MIC values through the broth microdilution method and the crystal violet assay, respectively. All EOs inhibited the growth of (1.25 ≤ MIC ≤ 5 mg/mL) while the growth of and was only affected by EOs from , (MIC = 5 mg/mL) and (MIC = 1.25 mg/mL) and (MIC = 5 mg/mL), respectively. In , most EOs induced a four- to sixteen-fold reduction in the MIC values of ampicillin, ciprofloxacin, ceftriaxone, gentamicin, and streptomycin, while in such a reduction is observed in combinations of ciprofloxacin with , , , , and , of gentamicin with , and of tetracycline with and . A smaller effect was observed in , in which only reduced the concentration of tetracycline four-fold. EO-antibiotic combinations also inhibit the biofilm formation. More precisely, all EOs with ciprofloxacin in , tetracycline in , and gentamicin in showed the highest percentage of inhibition. Combinations induce up- and down-methylation of cytosines and adenines compared to EO or antibiotics alone. The study provides evidence about the role of EOs in enhancing the action of antibiotics by influencing key processes involved in resistance mechanisms such as biofilm formation and epigenetic changes. Synergistic interactions should be effectively considered in dealing with pathogenic microorganisms.